1. Field of the Invention
This invention generally relates to a module for packaging nuclear waste of various radiation levels which may be safely and permanently buried at a waste disposal site.
2. Description of the Prior Art
Various means for packaging nuclear wastes are known in the prior art. One of the earliest types of packages used were steel-walled, 55-gallon drums. Such drums were used in the early "kick and roll" type waste burial systems. After they were packed, the surface radiation of such drums was often too high to allow them to be contact handled by human workers; accordingly, the packed drums were handled by long boom cranes. These cranes dropped the drums into a simple earthen trench, where they were buried. Unfortunately, the use of such 55-gallon steel drums in such trenches proved to be a highly unsatisfactory method for the ground disposal of nuclear waste. The loose packed soil which these trenches were filled in with was much more permeable to water than the densely-packed soil which formed the trench sides, or the dense rock strata which typically form the trench bottom. Consequently, the relatively loose and water permeable soil which surrounded the drums cause these trenches to collect large amounts of standing water in what is known as the "bathtub effect". This standing water ultimately caused the steel walls of the drums buried within these trenches to corrode and collapse. The collapsing drums and compaction of the soil over time in turn resulted in a downward movement or subsidence of the soil which caused a depression to form over the top of the trench. This depression collected surface water and hence worsened the tendency of the trench to collect and maintain a pool of standing water over the drums. The resulting increase in standing water resulted in still more subsidence and accelerated the corrosion and collapse of the drums buried therein. The corrosion and collapse of the drum containers at such sites has resulted in some radioactive contamination of the ground water flowing therethrough.
To solve the problems associated with the drums used in such "kick and roll" packaging and disposal systems, packages having relatively thick, radiation-shielding and water-impermeable walls were developed. In contrast to the thin walls of the 55-gallon drums, the thick walls of these concrete packages reduced the surface radiation of the resulting package to the point where they did not have to be handled by long boom cranes, but could instead be safely handled by human operators. Additionally, the thick layer of concrete was much more resistant to degradation from ground water. In use, these thick-walled concrete packages were carried to the sites where waste was generated, which was typically a nuclear power plant. The waste was thrown directly into the interior of these packages, and the packages were sealed on-site. The sealed packages were then carried to a remote disposal site and buried. The low surface radiation associated with these concrete packages allowed them to be stacked in an orderly fashion within the burial trench by shielded forklifts.
Despite the superiority of such concrete packages over the drum-type packages used in "kick and roll" systems, there are still a number of shortcomings associated with this particular form of packaging. First, these particular packages could not conveniently handle high-level wastes, such as spent control rods; the concrete walls of the packages were simply not thick enough to reduce the surface radiation of the package to an acceptable level. A second, related problem was that the surface radiation of the resulting packages varied depending upon the activity of the particular waste packed therein. Since it is always desirable to surround the "hottest" packages under the least active packages in the burial trench, the fact that the surface radiation of these particular packages varied over a broad range made it difficult to ascertain the optimal order of stacking. Third, these packages effectively had only a single radiation and water barrier between the waste contained therein and the outside earth. If the concrete walls of these packages became cracked or broken due to seismic disturbance, there were no backup water or radiation barriers. Fourth, these concrete packages were not conveniently recoverable from the burial site. This last shortcoming is a particularly serious deficiency if seismic disturbances cause a particular package to crack or rupture to the point where radioactive matter may be leached out of it. The inability to selectively recover a particular package may necessitate a massive digging-up and relocation of the burial site.
Clearly, a need exists for a ground-disposable nuclear waste package which is capable of packaging radioactive waste of varying levels of radioactivity while presenting the same or at least similar levels of surface radiation for such wastes. Ideally, such a package should surround the waste contained therein with multiple water and radiation barriers should the outside walls of the package crack or break for any reason. Finally, the package should be stackable into a configuration which is highly resistant to damage from seismic events or other natural disturbances, and should be easily recoverable should any particular package in the stack become damaged.